How do I find the cheapest flights with flexible dates and routes?

Plan a 7-day Spain and Portugal itinerary with trains and must-see cities.

How can I search route and date combinations for the cheapest fares including nearby airports?

Where can I find multi-country Europe itineraries with lodging and flights included?

Which Japan travel-planning app is best for a 10-day Tokyo–Kyoto–Osaka itinerary?